1. Field of the Invention
The present invention relates to a light polarization control element using an stress-optical coefficient glass and a method for producing the stress-optical coefficient glass used for the light polarization control element.
2. Description of the Related Art
A low stress-optical coefficient glass is applied to, for example, a light polarization control element or the like. Such an element is exemplified by an optical element (optical parts) such as a substrate and prism body constituting a polarizing beam splitter or a space light modulating element for conducting a polarized light modulation.
In the low stress-optical coefficient glass, it is a necessary requirement that a birefringence, which is caused when an external mechanical force or thermal action is applied thereto, should be small. In other words, an stress-optical coefficient thereof should be low. Further it needs to satisfy the following requirements: it should have a predetermined refractive index and a low liquid phase temperature; it should be manufactured easily enough to be mass-produced; it should have a certain transmittance corresponding to a predetermined wavelength; and it should not cause any environmental pollution. As for the refractive index, the transmittance and the like among these requirements, the desired characteristics are varied according to their actual uses. For example, with respect to a polarizing beam splitter in a liquid crystal projector, it is preferable that the stress-optical coefficient falls into the range of +0.8.times.10.sup.-12 Pa to -0.3.times.10.sup.-12 Pa, and the refractive index is within 1.57 to 1.73.
In the liquid crystal projector, a birefringence (an stress-optical coefficient) becomes a problem for the following reasons.
In the case of a transmitter type liquid crystal projector, the reasons are as follows;
The number of pixels in a liquid crystal tends to increase in order to comply with the tendency for a finer image and to match up the number of pixels of a personal computer. Specifically each type of the VGA, SVGA and XGA uses 640.times.480 pixels, 800.times.600 pixels and 1024.times.768 pixels, respectively, with the XGA type now occupying the mainstream position among them. Such a trend to a larger number of pixels is expected to proceed further, and the SXGA (1080.times.1024 pixels) is expected to appear in the near future. PA1 A single pixel consists significantly of an opening portion for transmitting a light beam therethrough and a transistor portion for driving the pixel, wherein the transistor portion does not allow any light beams to pass therethrough. The larger the number of pixels in a liquid crystal becomes, the smaller the area of a single pixel. However, there is a limit in decreasing the size of a transistor existing in the single pixel. Further, as the number of pixels is increased, there is increased the ratio of the area occupied by the transistor portion where light beams are not transmitted (referring to FIG. 1(a)-1(c)). Consequently, the ratio (an opening ratio) of light beams transmitted through the liquid decreases, and hence one is forced to use a means (such as a high output power) to radiate a stronger beam than that of a lamp in order to prevent the brightness in the liquid crystal from decreasing. As a result, there appears a thermal distribution in the PBS prism polarizing the P-wave and S-wave of the light beam, and thereby a thermal stress affects the inside of the glass. PA1 FIG. 2 is a simplified structural view of the reflector type liquid crystal projector. Under its reflecting surface, the reflector type has a transistor to drive the liquid crystal, which is different from the constitution of the transmitter type. As a result the reflector type has the characteristic that the opening ratio never decreases even when the pixel used therein keeps up with the trend to a finer image. Thereby the reflector type has an advantage to comply with an increased number of pixels in future liquid crystal projectors. However, a fault of the reflector type is that the optical path length gets longer because a light beam goes and returns within a PBS prism 1 and a cross prism 2. When an optical path difference due to a birefringence is denoted by .delta. (nm), there exists the following relation. EQU .delta.=B.times..sigma..times.d PA1 P.sub.2 O.sub.5 ; 20-60 wt % and PA1 BaO+PbO; 40-73 wt %. PA1 P.sub.2 O.sub.5 ; 30-60 wt %, PA1 BaO; 10-60 wt %, PA1 PbO; less than 50 wt % and PA1 BaO+PbO; 40-70 wt %. PA1 P.sub.2 O.sub.5 ; 35-56 wt %, PA1 BaO; 12-56 wt %, PA1 PbO; less than 46 wt % and PA1 BaO+PbO; 43-65 wt %. PA1 P.sub.2 O.sub.5 ; 20-70 wt % and PA1 BaO; 30-60 wt %. PA1 P.sub.2 O.sub.5 ; 30-60 wt % and PA1 BaO; 40-60 wt. %. PA1 B.sub.2 O.sub.3 ; less than 4%, Al.sub.2 O.sub.3 ; less than 3%. Nb.sub.2 O.sub.5 ; less than 2%, WO.sub.3 ; 0-6, MgO; 0-5%, CaO; 0-6%, SrO; 0-6, ZnO; 0-6%, La.sub.2 O.sub.3 ; 0-3%, TiO.sub.2 ; 0-5%, Li.sub.2 O; less than 1%, Na.sub.2 O; less than 3%. K.sub.2 O; 0-3% Cs.sub.2 O; 0-3%, Li.sub.2 O+Na.sub.2 O; less than 5%, Na.sub.2 O+K.sub.2 O; less than 3%, Sb.sub.2 O.sub.3 ; less than 0.5%, As.sub.2 O.sub.3 ; 0-2%, SnO.sub.2 ; 0-2%, Sb.sub.2 O.sub.3 +Bi.sub.2 O.sub.3 +Tl.sub.2 O.sub.3 ; less than 0.5%. PA1 P.sub.2 O.sub.5 ; 23-42 wt %, PA1 PbO; 50-70 wt % and PA1 Nb.sub.2 O.sub.5 ; 0.5-5 wt % (as an optional addition) PA1 P.sub.2 O.sub.5 ; 25-40 wt %, PA1 PbO; 52-71 wt % and PA1 Nb.sub.2 O.sub.5 ; 1-4 wt %. PA1 MgO; 0-10%, CaO; 0-10%, SrO; 0-10%, BaO; 0-15%, B.sub.2 O.sub.3 ; 0-10%, ZnO; 0-8%, Sb.sub.2 O.sub.3 ; 0-2%, As.sub.2 O.sub.3 ; 0-2%, SnO.sub.2 ; 0-2%. PA1 ZnO; 0-8 wt %, Al.sub.2 O.sub.3 ; 0-5 wt %, La.sub.2 O.sub.3 ; 0-3 wt %, MgO, 0-10 wt %, CaO; 0-10 wt % and SrO; 0-10 wt %. PA1 P.sub.2 O.sub.5 ; 0.029=0.01, BaO; -0.021.+-.0.01, PbO: -0.036.+-.0.01, B.sub.2 O.sub.3 ; 0.05.+-.0.01, Al.sub.2 O.sub.3 ; 0.01.+-.0.01, Nb.sub.2 O.sub.5 ; 0.11.+-.0.01, WO.sub.3 ; 0.05.+-.0.01, MgO; 0.04.+-.0.01, CaO; 0.016.+-.0.01, SrO; 0.008.+-.0.01, ZnO; 0.037.+-.0.01, La.sub.2 O.sub.3 ; -0.01.+-.0.01, TiO.sub.2 ; 0.03.+-.0.01, Li.sub.2 O, 0.015.+-.0.01, Na.sub.2 O; 0.025.+-.0.01, K.sub.2 O; 0.03.+-.0.01, Cs.sub.2 O; 0.03.+-.0.01, Sb.sub.2 O.sub.3 ; 0.04.+-.0.01, Bi.sub.2 O.sub.3 ; 0.05.+-.0.01, TeO.sub.2 ; 0.05.+-.0.01, PbF.sub.2 ; -0.03.+-.0.01, BaF.sub.2, -0.015.+-.0.01. The constituents and the contents thereof are determined based upon these inherent stress-optical coefficients values and then a stress-optical coefficient glass having a desired stress-optical coefficient is produced.
In the case of a reflector type liquid crystal projector, the reasons are as follows:
In the above equation, the symbol .sigma. (10.sup.5 Pa) represents an internal stress when a thermal action or dynamic force is applied, the symbol d (cm) designates an optical path length, and the symbol B denotes a stress-optical coefficient (10.sup.-12 Pa). When the stress-optical coefficient B is constant and the internal stress .sigma. or the optical path length d becomes larger, the birefringence gets larger. When the birefringence increases, the resolving power into P-polarized light and S-polarized light is resultingly disturbed. In particular, under the condition of an OFF state representing a black color as a pixel, a light beam which is expected to have been converted from P-polarized light to S-polarized light remains P-polarized light and thereby produces an unevenness of the black color on the screen (in the transmitter type, the internal stress .sigma. and the optical path length d each becomes larger, which causes a problem).
Since a birefringence is presented by the product of the B, .sigma. and d, it is possible to decrease a birefringence by counterbalancing the increment due to the .sigma. or d, by the decrement of the B. Going to extremes, as long as the B is zero, the .delta. is zero even when the .sigma. or d is large.
In Japanese Laid-Open Patent Publication No. Hei 9-48631, a low stress-optical coefficient glass of B.sub.2 O.sub.3 --Al.sub.2 O.sub.3 --PbO system is described. In the glass, the constituent PbO has the strongest effect to decrease an stress-optical coefficient among the addable constituents to the glass. Accordingly, it is possible to make the stress-optical coefficient close to zero by using a large quantity of PbO in the glass, even though the components B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 each has the effect to increase the stress-optical coefficient. However, there is a demerit in that the transmittance around the wavelength of 400 nm is lowered when a large quantity of PbO is used.
In Japanese Laid-Open Patent Publication No. Hei 9-48633, a low stress-optical coefficient glass being fluorophosphate-based is described. The glass contains fluorine, which is accordingly volatilized in quantity during the time of dissolution. Consequently it is difficult to obtain such a glass which a high reproducibility concerning optical characteristics such as an stress-optical coefficient, dispersibility and the like. Also, striae within the glass are developed influentially to thereby reduce the yield of the good glass products. Hence, to contain the constituent of fluorine does not necessarily mean to lower an stress-optical coefficient, especially since the use of a large quantity of fluorine causes a bad homogeneity within the glass.
In Japanese Laid-Open Patent Publication No. Sho 50-71708, a phosphate optical glass of P.sub.2 O.sub.5 --PbO--Nb.sub.2 O.sub.5 system is described. This glass is intended to obtain an optical glass, which gets little colored and has a high refractive index, and is never oriented to be used in an application for a low stress-optical coefficient glass. Consequently, in the most of the examples no less than 5 wt. % of Nb.sub.2 O.sub.5 is contained therein and hence the stress-optical coefficients thereof are over +0.8.times.10.sup.-12 Pa. In each example using less than 5% of Nb.sub.2 O.sub.5, since more than 50% of PbO is contained therein, the refractive index thereof becomes over 1.73, and an external transmittance is occasionally worsened because a large amount of PbO is used.
In Japanese Laid-Open Patent Publication No. Hei 2-188442, a phosphoric acid based optical glass of P.sub.2 O.sub.5 --Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --RO (Mg, Ca, Sr, Ba, Pb) system is described. This glass is intended to obtain an optical glass, which has a high light beam transmittance in the ultraviolet region, and is never oriented to be used in an application for a low stress-optical coefficient glass. Consequently, in the glass having more than 36% of P.sub.2 O.sub.5, the stress-optical coefficient is more than +0.8.times.10.sup.-12 Pa because the sum of the constituents of BaO and PbO is less than 42%. When the constituent of P.sub.2 O.sub.5 falls into the range of 32 to 36%, more than 4% of Al.sub.2 O.sub.3 is contained therein and accordingly the liquid phase temperature becomes higher. As a result this is inappropriate for an actual manufacturing thereof.
Further, In JOURNAL OF THE SOCIETY OF GRASS TECHNOLOGY (1957, 359T-362T, "The effects of the Polarization of the Constituent Ion on the Photoelastic Birefringence of the Glass", BY MEGUMI TASHIRO), a binary glass of P.sub.2 O.sub.5 --PbO system is disclosed, and it is described that the stress-optical coefficient becomes zero when the composition consists of P.sub.2 O.sub.5 : 40.5 wt. % and PbO : 59.5 wt %. However, the glass of such a composition has a drawback that it is poor in the chemical durability, liable to cause yellowing due to moisture in air, and its practicability is low.
As mentioned above, there has not yet been obtained a low stress-optical coefficient glass which overcomes the above various problems and which has a stress-optical coefficient, a refractive index and the like which falls within a predetermined range, and which glass further has a low liquid phase temperature and the possibility of being mass-produced.